


lessons in monster biology (or how to get a skeleton off)

by MageOfLight



Category: Undertale
Genre: Aged-up Frisk, Body exploration in the name of Science, Ecto-Tongue, Exploration, Frisk (or reader) is a dirty sneak, Gender-Neutral Frisk, Gender-neutral Reader, Science?, Shameless Flirt, cool. let's do that then., even though i did not include an underage archive warning?, frisk is a young adult, magic tongue, man i don't even know what i'm doing with these tags, so apparently i need to note in my tags that frisk is a legal adult in this?, this is so self-indulgent
Language: English
Status: Completed
Published: 2016-09-18
Updated: 2016-09-18
Packaged: 2018-08-15 18:32:26
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 5,037
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/8068243
Author URL: https://archiveofourown.org/users/MageOfLight/pseuds/MageOfLight
Summary: Okay, so, this is really, REALLY self-indulgent. Based on Snassty's NSFW audio, 'Sans gives you a lesson in monster biology'. Technically Frisk, because I ship Sans/Frisk like hell, but since I'm not using their name at all and tried to not put in any defining features, it could be Sans/Reader, too....completely unbeta'd, so I'm sorry if anything slipped through when I was scanning through it. I tried to get all the errors, but it was mostly written in the middle of the night when I wasn't likely to be bothered, so... (Feel free to point out any weird bits. I can always fix them.)





	

**Author's Note:**

  * Inspired by [Sans gives you a lesson in monster biology](https://archiveofourown.org/external_works/230260) by snassty. 



The key shook in your hand as you slipped it into the lock. You weren't sure it was even for this door, but you hoped, having been curious about it since the moment you saw the door in the first place. Honestly, you were rather nervous. You had no idea what you would find behind the door if the key worked... or possibly how mad Sans would even be at you if it did. The key had been hidden in his room after all. Sure, he might have given you the key to his room in the first place, but you were positive it was all for a prank, what with the treadmill in the dark room (and how had that even worked anyway? You hadn't even heard the stupid thing, and every way you turned, the treadmill seemed to have been facing the same direction as you. You blamed Sans for that, too).  
  
Well, even if he was mad at you for sneaking into... whatever was behind the door, you could always use the excuse of getting revenge for the prank. Not a very _good_ excuse, mind you, but perhaps it would work. You shook off your thoughts and attempted to turn the key. To your surprise, you heard the click of the lock disengaging. "...huh. Didn't think that would actually work..." you mutter to yourself. You remove the key, tuck it away in your pocket to put back later, then open the door.  
  
The light from Snowdin shines onto a long staircase, and your gaze flicks around for a light switch. It's very dark toward the bottom, and you don't want to go down there without being able to see. You find it and flick it on, illuminating the rest of the staircase and a small hall at the bottom. Muttering to yourself about only taking a quick peek and then getting out, you venture down the stairs almost nervously.  
  
At the bottom, you turn the corner to see... what looks like it could be a tiny lab of some sorts. There's a counter lined with drawers with a sink at one end. The wall next to you has a dusty old love seat, big enough for Sans to maybe comfortably take a nap on (though honestly, he could probably comfortably nap anywhere, the lazybones), and at the other end by the sink is a large lump covered in a sheet. You can see what looks like a tool box poking out from the bottom of the sheet and wonder what exactly is under there.  
  
Your curiosity leads you to poke around, opening the drawers and looking inside. You find blueprints with strange symbols and handwriting that is so much chicken scratch that you can't even _begin_ to make out what it actually says. It's worse than a doctor's penmanship! You almost laugh, putting them back and looking in the next drawer. This one only contains a large pile of photos. Sans is in every single one, looking so _happy_ it almost hurts. In the entire time you've known him, you've never seen him looking like this. It brings a tear to your eye, and you wonder if maybe, you could do something to make him look like that again. The thought is only fleeting as you hear the door upstairs open and close again. There's the sound of muttering and then footsteps, accompanied by the slap of slippers. Sans.  
  
You drop the photos back in the drawer, close it quietly and attempt to go hide behind the sheet-covered lump in the corner, but before you can, Sans is at the bottom of the stairs. You stare at him wide-eyed, but he doesn't seem at all angry. In fact, he almost seems... amused?  
  
You start as he chuckles, the sound surprisingly weak compared to how he usually sounds. "well... it, uh, looks like you found my laboratory... or at least, that's what this place used to be. y'know... back when, uh," he takes an audible breath and lets it out as a gusty sigh, "well, things were, uh, different, to say the least." He pauses, looking around. He isn't quite looking in your direction, perhaps seeing something that isn't actually there anymore.  
  
You let out a breath you didn't realize you'd been holding as you realize that he really _isn't_ mad that you're down here. He doesn't react to the sound you'd made, and you continue to watch him for a moment as he stares off into space. His eyelights are little pinpricks, smaller than you've ever seen them, and the smile on his face just looks sad. He clearly misses whatever he had in the past and your thoughts go to the photos in the drawer.  
  
You decide you need to snap him out of his thoughts and ask the first thing you can think of. "Hey, Sans?" His attention shifts, eyelights returning to normal as he looks at you questioningly. "Mind if I look around a bit?"  
  
He looks a little startled, like maybe he's wondering why you're even asking, and you flush a little. You'd stolen the key and come down here without permission, after all. Still, he answers you, sounding a bit awkward suddenly. "yeah, you can look around. i don't care." His eyes scan the dusty room and he rubs the back of his skull in a gesture you could swear was almost... sheepish? His voice reflects the same as he speaks up again. "sorry for the mess n' stuff, i... haven't exactly been down here in a while." He shrugs, leaning against the counter, staring at the opposite wall now.  
  
The room isn't messy in your opinion. Especially not after seeing his bedroom before. It's just very dusty. You wonder how long it's been since he's been down there. It sounds like 'a while' is quite the understatement to you. You poke around some more, opening drawers and staring at bits of unintelligible writing. You wonder if Sans is the one that wrote all of this, or of it was one of the monsters in the photos. You don't open the photo drawer again, instead poking around in a couple cupboards where you find little bits of machinery that you couldn't even begin to figure out what it's all for. Honestly, you've always been pretty nosy, and now that you actually have permission, you're not going to pass up the opportunity to let your curiousity get the better of you.  
  
While you're poking around, Sans wanders over to the sheet, fiddling with something under it. You're drawn away from what you're looking at as he makes a contemplative sound, and you glance over at him. He's no longer looking at what he's doing, but watching you. You tilt your head but ignore it, going back to poking around. This is his space after all. He's probably making sure you don't break anything.  
  
A little later, you're once again drawn away from fiddling with another little piece of machinery as you hear Sans sigh and make a little humming sound behind you, and you look back over at him as you put the gadget back. It takes him a moment to realize your attention is entirely on him now, and he makes a startled sound as he locks eyes with you, his cheekbones flushing blue. Interesting. Must be his magic causing that blue flush. You briefly entertain the idea of seeing how blue you could make him turn, but shake off the thought as Sans lets out a tiny, almost nervous sounding chuckle. "s-sorry, kiddo, i uh..." He clears his throat, making you wonder how that even works when he doesn't even _have_ a throat in the first place. "i don't mean to stare or anything. i-it's just. somethin' about being down here gets me into the mood to... observe, y'know? i-i mean..." He trails off a moment and you tilt your head, attentive. You've never seen him as anything beyond laid-back and quietly confident. This is _new_ , and your curiousity is peaked. He's sweating and not quite looking at you now. If you didn't know any better, you'd say he was being _shy_ of all things.  
  
"i'm curious about you. you're new and different and..." You watch as he seems to shiver a little, his bones making a slight rattling sound as he lets out a shaky breath, drifting a little closer to you as he breaths out the next word. "...exciting..." You stand from where you'd been crouched down, suddenly, painfully interested in the attention he's giving you right now. This is way more interesting than the dusty contents of the lab, and you find yourself paying rapt attention to his expression. He's still sweating, probably nervous about how you're taking this as your own expression is carefully blank. You've always been really good at holding a poker face, and you're employing that talent right now. Just as he looks like he's about to maybe back off, you grin, stepping closer to him. Frankly, you'd been curious about him since the moment you met him, and seeing as he'd just blatantly stated as such about you, you figured a little... _exploration_ was in order about now.  
  
You calmly take his hand, bringing it up against your side with a cheeky, flirtatious smile, and he gives you this startled, flustered look. "wha-... oh. i-i see. you're uh, letting me take the hands-on approach, are you?" he asks, and you nod. He chuckles softly, his usual easy confidence seeming to return, though he's still flushed in the face. It looks even better on him now that he's more relaxed. You like it. "good. i learn better this way." This time it's your turn to flush slightly, your cheeks going pink as his hand slides up your side, your sweater being hiked up slightly at the movement. Not one to just stand there and passively let him explore you, you lift one of your hands and brush your fingers over his cheekbone. It's a lot warmer than you expected, and your fingers tingle as they move across the flushed bone. Definitely magic. He makes a surprised sound as you touch his face, his breath coming out shaky as you glide your fingers down his face to his neck, brushing against the vertebrae. "oh... y-yeah, i guess i'm just as unfamiliar to you as you are to me, huh?" His voice is as shaky as his breathing, and admittedly, it's pretty hot.  
  
"...you don't mind, do you?" you ask, fingers stilling on his neck. You probably should have asked first, but if your sneaking into his lab in the first place is anything to go by, you're more of an 'act first, ask later' kind of person. It's a bad habit, and you know you really need to break it. You're not a child anymore (even if Sans does have this thing with calling you 'kid' or 'kiddo'. It's probably the striped sweater), so you really have no excuse for that kind of behaviour.  
  
He lets out a breathy sound at your question, and probably the fact that your hand is still on his neck. "n-no, i don't mind. we can explore each other... i-if that's okay." You can't help but think the stuttering is really adorable. You grin.  
  
"I don't mind," you echo, eyes purposely, flirtatiously, going half-lidded. You are a shameless flirt, and by this point, everyone knows it. You cannot bring yourself to care, considering the fact that his face has just turned a little more blue. That's adorable, too. You continue running your hand along his neck, drawing another breathy sound from him. You could get used to those kinds of sounds fairly easily.  
  
"good," he murmurs, and you realize his smile is not quite as fixed as you'd always thought it was. It was a little more lax now as he shifted slightly away, giving you a once over as if deciding how he was going to start his own exploration. Well, you had kind of derailed him before, so you stepped back, leaning casually against the counter. You'd let him get the ball rolling this time, once he figured out where to start.  
  
You watch as he steps closer to you again, his hands coming to the bottom of your sweater, shifting it up slightly before stopping and looking at your face. At the wordless question, you flush once again and reach down yourself, grasping the bottom and tugging it off. His hoodie joins your sweater on the counter, and then he's looking at you properly. His eyelights seem to blow a little wider at the sight of you shirtless, and you find yourself blushing a little more than before as his eyes sweep over you. There's definite interest in his voice as he speaks up finally, and you find yourself focusing incredibly closely to what he's saying. "you know, it's... it's interesting how we're so very alike, you and i..." His voice shifts lightly, lowering as he reaches to touch you. "and yet so... strikingly different. your basic internal structure is nearly identical to mine, but then you have all of these parts that i don't." His hands trail over your chest, down your stomach, one leaving your torso to trail down your arm, fingertips skating down to your hand, taking it and examining it.  
  
"organs... fingernails..." His voice is soft now as he touches you, fingertips tapping against one of your nails with a quiet click as he says the word. His other hand isn't idle, either, raising to brush gently over your scalp and through your hair, causing you to shiver. "hair... flesh..." he leans in as he says the last two words, voice nearly a whisper, and you find yourself pressing slightly into him, letting out a rasping, breathy sigh. His hand is still combing through your hair. It feels good. You'd always liked having your hair played with.  
  
"i wonder if... we're sensitive in the same places?" His hand leaves your hair and you bite back a disappointed sigh as it ghosts down your neck. The fingers of his other hand thread through yours as he continues. "i mean obviously there are places on me that i can't touch on you, like these... tender spaces in between your metacarpals..." Fingers gently press against the skin on your palm at the words. "but i wonder about things like..." he trails off, hand abandoning yours to return to your torso, resting on your hip as his other hand ghosts further down, his voice dropping to what could only be described as a purr. "your clavicle... sternum... the base of your spine..." The fingers of one hand brush over the first two spots, his other hand moving to rub over your spine and you find yourself arching into his touch. You hadn't known you were sensitive there, but he seems delighted at your reaction, grinning wider. "oh. you _are_ sensitive there." The words come out teasing, and you narrow your eyes. That was entirely enough standing there passively for you.  
  
Not one to be out-done, you reach out both hands, teasingly tracing across his clavicle. The reaction is instant and entirely exciting. He arches with a soft, gasped cry. "mm... yeah, so am i..." he manages to get out before moaning softly as you move one of your hands to his back, running it down his spine through his t-shirt before your hand wraps around the base of it. "ohh, especially there...!" The words are gasped and breathless. You give him a pleased grin, letting go and wrapping your hands around the bottom hem of his shirt.  
  
You look at him questioningly and he lets you pull his shirt up and off, shooting you an unsure smile as you run your eyes over his bones. It's strange, but... not unpleasant, either. "ah, you really are curious about how i look, huh? heh... can't say i don't feel the same." You grin at that. What an understatement. You were both shirtless in a cool, basement lab, 'observing' each other. You almost laugh as you drop his shirt on the small pile on the counter, wondering just how much clothing is going to come off in the name of 'science'. You move both your hands to his ribs, light touches ghosting along the bone and occasionally curling into the gaps to feel the inside. His breathing is shaky as he lets you explore, and that's a pretty nice sound, too. You wonder how he's so receptive to touch when he doesn't have any nerves to feel this with. "oh, stars... you. are. in. credible," he breaths, and you stop, looking at him with what is probably the most skeptical look you've ever given anyone. He seems to be able to read your face pretty well as he chuckles lightly, cupping your face with his hand. You nuzzle into it thoughtlessly. "no, i'm serious! you are," he tells you, laughter still in his voice. His hand leaves your face, trailing down your neck and lightly massaging. You feel your pulse jump at that. It feels good.  
  
"so soft... warm... pulsing with the blood in your veins..." His voice drops lower, and you shift, tilting your head a little to give his fingers more room to work. Your eyes drifting closed only to snap open moments later at the feel of... something that isn't bone against your skin. It's soft and has the same tingling quality as the magic flushing his cheek bones. It drags across your skin and you draw in a shuddering gasp. "mm. salty, yet sweet..." you hear him murmur, and finally look at him properly. His mouth is open. Oh stars, his mouth is open and that is a tongue. You've never seen his mouth open before. Not properly, anyway. A little bit when he's drinking ketchup, but never like this. His tongue is blue and faintly glowing as it darts out to lick you again, and you shiver as tingles run along your skin. Fuck, that's nice. He stops, though, looking embarrassed, and you bite back a whine, watching him and wondering what's going on in that skull of his as he draws in a breath. "s-sorry, this is... this is getting very unprofessional, isn't it?" He's clearly nervous, his voice shaking a little and faster than his normal drawl.  
  
You give him a moment to stew before laughing softly. "Sans," you say, grinning as you suddenly have his full attention. "If I had a problem, I'd tell you to stop. This? This is nice. We can keep going," you tell him. You don't normally speak a lot, but he seems to need the reassurance at the moment, so you don't mind that. He still looks unsure, though, so you roll your eyes and lean in, nipping at his clavicle. Once, twice, three times.  
  
He jerks at each nip, and you bite a little harder than you meant to on the third, causing him to cry out. "ah-! mmh...! that feels...!" You pull back with a tiny mutter of, "Shit..." staring at him. His face is really blue now, and you stare at him worriedly, fingers rubbing at the bone apologetically. "ah, y-you can... you can bite a little harder, i don't mind," he tells you, voice shaking.  
  
"...that... didn't hurt?" you ask, brows raised, though your own voice is shaking slightly, worried. The last thing you want is to have hurt him at all. Above all else, he's your friend and you care about him a great deal. Possibly more than any of your other monster friends.  
  
"n-no, it takes a lot to hurt me... that feels good," he replies, and you let out a soft, relieved sigh. He doesn't sound hurt. In fact, his voice is shaking in the most delightful way as you continue to rub the bone you'd bit.  
  
"O-okay... tell me if I hurt you at all," you whisper, leaning in to nip at his clavicles again. This time, you dig your teeth in a little harder with each nip. He doesn't protest. In fact, his breathing gets shakier, soft gasps and moans nearly punching the breath out of your chest. The bone doesn't really have much of a taste as you lick over each bite, but there's that ever present tingle of magic that seems to light a fire inside you. It's really nice. One of his hands is braced on your shoulder, fingers digging in slightly in a rather pleasant way, and you grin around the bone. He's really very responsive.  
  
The next few minutes are a rush of exploration, blurring together in your mind to be sorted out later. Your shorts somehow wind up on the ground. You carelessly kick them off to one side, leaving you in just your boots and underwear. You're not sure how that happened, but you don't particularly care, either. His hands are trailing paths of fire along your skin, and his mouth isn't idle, either, licking at your neck in such a delightful way that you moan, soft and low. He licks again, drawing another moan from you, your head tilting back to give him better access. "ohh... the way your throat vibrates when you moan..." he whispers, and you let out a shaking breath, hands rubbing down his spine and across the inside of his ribs. "oh, that's very nice, do that again..." His breath is hot against your skin, tingling across the spots he'd been licking before, wrenching another moan from you. You do as commanded, caressing the bones under your hands again.  
  
He licks you again before nipping at the skin under his mouth, and you shiver, a little breathless cry tugged from you. He exhales against you, causing another series of shivers. "mm, i can feel shivers dancing across your skin," he murmurs, then lifts his head to whisper right in your ear, "am i doing that?" He chuckles right into your ear. Cheeky asshole. He _knows_ he's doing that. In response, one of your hands wraps around his spine, fingers massaging and wrenching a surprised moan from him. You grin. Ah, revenge is so sweet. His hands brace against the counter behind you, and you take a moment to observe him. Most of the 'exploration' thus far had been confined to the torso. You wonder if he'd let you go lower. Your hands shift to the waistband of his shorts before you look back at his face.  
  
"Can I?" you ask, canting your head slightly with a questioning look, fingertips catching slightly at the tops of his iliac crests. The sounds that draw from him are very, very nice.  
  
"nnh... yeah, you can..." he pauses to draw in another breath as you tug his shorts down at the obvious permission, "you can touch me there..." You grin, hands brushing against the bones of his pelvis, watching as he visibly swallows back a moan in order to keep talking. "c-can i...?" His own hands are at your hips and you know exactly what he's asking.  
  
You lean forward, hands pausing as you stop with your mouth close to where his ear would have been. "Yeah, go ahead," you whisper, then drop your head to his shoulder, nipping at the bone there as your hands massage and rub at each bone they come across on his lower body. He's obviously enjoying it, hands squeezing at your hips and breathless gasps and moans escaping him as you work at him. And then his own hands are moving, rubbing at you through your underwear and you let out a choked moan, hips rutting forward into his hands. "ohh. oh, that's... that's new, thaaat's very new..." he mutters, obviously very effected by your response to his touches. You hadn't even realized you'd been that worked up, but now that you're aware of it, all you want is for him to touch you more. "i, uh... i might need to explore that some more." He's speaking more to himself than you at this point, so you don't respond, simply trying your best to focus on reciprocating his touches, trying to give as good as you're getting. And then his hands stop and you can't stop the whine that escapes you before your realize his hands are at the waistband of your last bit of clothing. "may i...?" he asks, and fuck, if that's not hot. You carelessly kick off your boots as you nod, and your underwear soon joins the cast-off clothing on the floor.  
  
His eyes are on you now, staring at your groin and you turn bright red at his attentive gaze. He's staring at you like you're a puzzle to solve or something, and honestly, it's more than a little embarrassing, but you put up with it because obviously, he's never seen this before. The staring goes on a little too long, though, and you shift uncomfortably. This seems to snap him out of it, and he grins sheepishly at you before... oh fuck, lifting you onto the counter as if you weighed nothing at all. He grins at your surprised face as you stare at him, but then you notice the tingle of magic flickering over your skin, and you realize how he did that. You're blue now. Your soul pulses with his magic, coloured a vivid blue instead of it's normal red. What a lazy cheater. You can't be mad at him, though. It's just who he is, and you lo-... like him just the way he is. The magic fades from you and your soul shimmers from view, removing all distraction as he opens his mouth. His tongue darts out, licking impishly along his teeth and you feel a bolt of arousal shoot down your spine at the sight, making up for his earlier staring. No more words are exchanged as he lowers his mouth to you, tongue tracing along you as if you were some sort of amazing treat. It feels utterly amazing, and you soon find yourself hunching over him, one hand braced on his shoulder, the other cradling the back of his head as choked moans and cries wrench from your chest.  
  
The magic of his tongue tingles and sends fire racing through your veins, and it isn't long before your release hits with a sharp cry of his name. He draws it out, mouth not leaving you until you're shuddering in the aftershocks and pushing him away because it's starting to hurt, your skin oversensitive to the touch of his tongue. You sit there panting for a few minutes, just coming down off the high before you grin at him. He looks far, far too pleased with himself at the moment. Time to return the favour. You hop off the counter (carefully, because you're well aware that your legs are probably more like jelly at the moment) and push him against the counter with an impish smile of your own as you drop to your knees in front of him. He looks startled as he looks down at you, but not like he's going to protest, so lift your hands, brushing them slowly up his femurs. He stares down at you, eyes wide, but he's still not protesting.  
  
You lick your lips, and he gulps at the motion. If the room were dark, his face alone would probably light up the area. You laugh softly at the thought, and then lean in, licking at him. You explore the bones of his pelvis with lips, tongue, and teeth, wrenching cries and moans from him that send little shocks and shivers through you. You could probably get off on those sounds alone if you hadn't just had an amazing orgasm yourself. Looking up, you see his head is thrown back, arms shaking as they brace against the counter in order to keep himself standing. You gulp, realizing you're the one who's done this to him. It's an amazing, heady feeling. You grin, hands moving from where they'd been braced on his hips. One drifts up to wrap around his spine, fingers digging in, and he chokes on a moan, breath stuttering. The other hand goes looking for the most sensitive bone in his pelvis you can find. It turns out to be his coccyx. Rubbing your fingers against it along with massaging his lower spine actually drags a scream from him. He stiffens against you, back arched, suddenly utterly silent. Looking up, you can see his mouth is wide open, tongue lolling out slightly and a trail of saliva escaping his mouth. Fuck, that's hot.  
  
You keep going until he's scrabbling at you, fingers tearing frantically at yours in an attempt to get you to stop. You stop, drawing your hands away, only to dart back to catch him as his legs buckle. He sags against you, panting silently, and you realize belatedly that he was so blissed out he couldn't even speak. Holy fuck. You find yourself grinning brightly as you hold him close to you, not even caring that his ribs are digging uncomfortably into your chest. You're careful not to touch any spot that was sensitive before, but still, he shudders weakly against you every time you shift lightly, over stimulated and so wrecked he still can't make a sound. His breathing slowly evens out, but he's still slumped bonelessly (heh) against you, and you realize with a tiny shock that he's actually passed out. You laugh softly at this realization, then slowly, carefully stand up with him, coiling your hands around his femurs to keep him from falling as you make your way over to the couch. You could use a nap, too.  
  
Settling down with Sans laying against your chest, your head pillowed against the arm rest, you can't help but smile and pet his skull. He shifts against you, murmuring, and you let out a soft, fond sigh. You really do love this skeleton. Even admitting it in your mind causes you to blush, but you smile a little more anyway. You two can talk properly about this later, when you're both awake. For now, though, you close your eyes, relaxing and letting sleep take you.


End file.
